1. Field of the Invention
The present invention relates to durably hydrophilic synthetic substrates. More specifically, the invention relates to the use of durably hydrophilic synthetic substrates such as fibers and particularly staple fibers for the manufacture of wet-lay non-woven articles and paper, and articles made from these synthetic substrates.
2. Discussion of the Background
The most widely used non-woven fabric is paper made from naturally occurring fibers such as wood pulp or cotton. With the development of synthetic fibers, there has been considerable interest in their use in making non-woven articles and sheet-like structures, using the simple processing steps and equipment commonly employed in paper making from natural fibers.
There are no significant problems associated with the use of synthetic fibers on conventional paper making machinery. The fibers may be slushed or slurried in a conventional manner to disperse the fibers in water. However, the refining of synthetic fibers and pulps is frequently avoided, because certain types of refining equipment can form fiber bundles or knots. These bundles can result in the formation of greas spots in the final sheet due to fusion during the calendering process. High pressures can fuse synthetic fiber bundles even without heat during calendering. Drying operations involving synthetic fibers are frequently more fascile than the drying of cellulose pulps due to the fact that synthetic fibers are generally hydrophobic in nature and have improved drainage and drying characteristics. Care must be taken however, not to use temperatures which are higher than the melting point of the synthetic fibers. Sizing, dyeing and filling operations can be performed in the normal fashion.
Synthetic organic fibers can be manufactured to meet specific diameter, length and physical properties. In addition, synthetic papers have the advantage of high wet strength, toughness, chemical durability, weather resistance and excellent dimensional stability.
Failure of synthetic fibers to replace natural cellulose fibers is due in large part to their hydrophobic nature. In order to use synthetic fibers having satisfactory properties successfully in the manufacture of synthetic or semi-synthetic paper on conventional paper-making equipment, it is essential that the synthethic fibers have a dispersibility in water similar to that of cellulose fibers i.e., they should have hydrophilic properties.
Synthetic pulps, filaments and fibers are all useful for the manufacture of paper articles. Synthetic pulps are designed to be blended in all proportions with wood pulp and can be used with conventional paper-making equipment. Common synthetic materials used in these pulps include high density polyethylene or polypropylene, and aramids, for example Kevlar and Nomex. Pulps prepared from other polymers are also known, e.g., aliphatic polyamides, polyvinylchloride, acrylonitrile homopolymers and copolymers with halogenated monomers, styrene copolymers and mixtures of polymers. Synthetic pulps have very irregular surfaces with many crevices and an almost film-like nature, in contrast to synthetic staples which are smooth rods of synthetic polymer. The surface area of synthetic pulp is quite large, which results in high scattering coefficients and gives rise to high opacity in papers made from these synthetic pulps. Additionally, synthetic pulps generally have lower densities than cellulosic pulps with the result that lighter weight papers can be made from these synthetics. Lighter weight papers represent an economic disadvantage, however, when paper is sold by weight units.
Although synthetic pulps are dispersible in water, they do not absorb water so their dimensions are not effected by contact with liquid water or water vapor. As a result, sheets containing synthetic pulp tend not to change dimension as relatively humidity changes. However, the hydrophobic nature of synthetic pulps frequently results in paper having a lower wet-strength than paper made from cellulosic pulp. Improved water dispersibility or bonding has been effected by precipitation of appropriate materials onto the surface of pulp fibers. Anionic-cationic colloidal complexes have been formed in the presence of synthetic pulp. Preferred complexes are poly(ethylene-co-acrylic acids)/polyethylene imine and carboxymethyl cellulose/melamine-formaldehyde resin. A general discussion of synthetic pulps can be found in "The Encyclopedia of Chemical Technology", Kirk-Othmer, Vol. 19, pp. 420-435, John Wiley & Sons, Inc., 1982.
Synthetic fibers have also been used to make paper products. The synthetic fibers may be in the form of continuous filaments or staple fibers and may be optionally mixed with cellulosic fibers and used on conventional paper-making machinery. In order to use synthetic fibers successfully in the manufacture of synthetic or semi-synthetic paper, it is essential that the synthetic fibers have a dispersibility in water similar to that of cellulose fibers which, due to their morphology and chemical nature disperse readily and homogeneously in water. Additionally, the synthetic fibers should have sufficient wet-strength to enable the use of conventional paper-making machinery.
In recent years, a considerable amount of effort has been expended in the development of synthetic fibers having hydrophilic properties. Much of this effort has been directed to treating synthetic fibers with hydrophilic polymeric materials or the development of synthetic fibers possessing hydrophilic chemical groups in the fiber itself. U.S. Pat. Nos. 4,167,548; 4,092,457; 4,002,796; 3,963,821; and 3,928,496 disclose some of these processes.
In general, these processes are directed toward the production of a specific type of synthetic fiber, and do not have general applicability to a wide variety of commercially available synthetic staple fibers. Their utility is therefore limited to the specific fibers involved. There continues to be a need for durably hydrophilic synthetic substrates and particularly staple fibers which are useful in the manufacture of non-woven and paper articles and which can be produced using commercially available staple fibers as a substrate. Non-woven and paper articles produced from virtually any type of synthetic staple fibers and which can be made on conventional paper-making equipment are not currently available and would be highly desirable.